The present invention generally relates to the field of ink jet printing and, more particularly, to the delivery of ink to ink jet print heads.
Ink-jet technology is relatively well developed. The basics of this technology are described by W. J. Lloyd and H. T. Taub in xe2x80x9cInk-Jet Devices,xe2x80x9d Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988) and in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No 5. (October 1988), Vol. 43, No. 4, (August 1992), Vol.43, No.6 (December 1992) and Vol. 45 No. 1 (February 1994).
In an effort to reduce the cost and size of ink-jet printers and to reduce the cost per printed page, engineers have developed ink-jet printers having small, moving print heads that are connected to large stationary ink reservoirs by flexible ink tubes. This development is called xe2x80x9coff-axisxe2x80x9d printing. In such printers the mass of the print head is sharply reduced so that the cost of the print head drive system and the over all size of the printer can be minimized. In addition, separating the ink reservoir from the print head has allowed the ink to be replaced as it is consumed without requiring frequent replacement of costly print heads.
Typically in off-axis printing systems, the ink is supplied from the reservoir under pressure to a pressure regulator located near the print head. The pressure regulator reduces the pressure of the ink and delivers the ink to the print head as required within the back pressure operating range of the print head.
One complication in the evolution of off-axis printing is the increasing need to maintain the variation in the back pressure of the ink at the print head to within as small a range as possible. Changes in back pressure greatly affect print density and print quality, and major changes in back pressure can cause the ink either to drool out of the nozzles or to de-prime the print cartridge.
There are several causes for such changes in back pressure. One cause is the inability of the pressure regulator to sufficiently follow the variations in back pressure caused by the operation of the print head. Another cause occurs when air is entrapped within the print cartridge and the print cartridge is subjected to changes in environmental parameters such as altitude, acceleration, and temperature. If the air entrapped in a print cartridge acts according to the Ideal Gas Law, PV=nRT, then any changes in any of these parameters will cause corresponding changes in back pressure.
Back pressure regulators for ink jet printers are further described in the following patents:
U.S. Pat. No. 4,422,084 entitled xe2x80x9cFluid Tank and Device for Detecting Remaining Fluidxe2x80x9d to Saito
U.S. Pat. No. 5,650,811 entitled xe2x80x9cApparatus for Providing Ink to a Print Headxe2x80x9d to Seccombe et al.
U.S. Pat. No. 5,844,577 entitled xe2x80x9cBack Pressure Regulator Ink Jet Penxe2x80x9d to Pawlowski
U.S. Pat. No. 5,872,584 entitled xe2x80x9cApparatus for Providing Ink to an Ink Jet Print Head and for Compensating for Entrapped Airxe2x80x9d to Hauck et al.
Back pressure regulators having ink bags with internal springs and fabricated from flexible film are described in the following patents:
U.S. Pat. No. 5,325,119 entitled xe2x80x9cVariable Rate Spring Ink Pressure Regulator for a Thermal Ink Jet Printerxe2x80x9d to Fong
U.S. Pat. No. 5,757,406 entitled xe2x80x9cNegative Pressure Ink Delivery Systemxe2x80x9d to Kaplisky et al.
Prior pressure regulators have been found to be too large for the new printers that are currently being developed. One recent design goal has been to reduce the size of pressure regulators by one half along the scan axis of the print headxe2x80x94that is, the left and right directions in which the print head scans across the printing media. In addition, it has been observed that if the pressure regulators are large, then the number of ink hues that can be accommodated in a conventional printer carriage is limited. In other words, to achieve photographic quality output, there is a need to provide at least six different ink hues in a printer in approximately the same amount of carriage space as is presently available.
However, the solution to the issue of reducing regulator size is more complex than merely scaling down prior pressure regulators. The internal mechanisms and levers in prior regulators need to be a minimum size in order to operate reliably, to achieve acceptable pressure tolerances, and to provide comparable functionality. These prior designs were found to be unusable when the dimension along the scan axis was substantially reduced.
Further, some prior pressure regulators used film bags that expand and contract in order to maintain constant back pressure to the print head. These bags are folded from sheets of film and are heat staked together. However, the edges of these bags are attacked by the ink, the layers can delaminate over time from this attack, and the print head can fail as a result.
It should be apparent from the foregoing that although there are many types of thermal ink jet back pressure regulators, there is still a need for an approach that markedly reduces the scan axis dimension while protecting the compliant film from failing by being attacked by ink and still providing the same level of regulator functionality.
Briefly and in general terms, an apparatus according to the invention includes a print head for ejecting droplets of ink on to a printing medium and a back pressure regulator for receiving ink from an ink reservoir and for delivering ink to the print head. The regulator has a compliant wall that responds to atmospheric pressure on one side and the pressure of the ink in the regulator on the another side. Within the regulator is a valve that is actuated by the wall, regulating the pressure of the ink delivered to the print head. Also within the regulator is a compression spring that simultaneously pre-loads the valve shut and urges the compliant wall against the atmospheric pressure.
In another aspect of the invention, an apparatus is provided with a print head having two arrays of nozzles and two back pressure regulators that independently deliver inks of different hues to separate arrays of nozzles on the print head.
An apparatus according to the invention also includes two valve assemblies. In one assembly a valve having an elongate stem, an attached disk orthogonal thereto, and an elastomeric valve seat bonded onto the disk is provided. An axle supports the valve for rotation and a valve nozzle is either blocked or unblocked by the valve seat through rotation of the valve about the axle. In the other assembly, a valve having an elongate stem and an attached elastomeric disk orthogonal thereto is provided. The assembly has a valve body having an opening through which the stem extends and a valve seat surrounding the opening. There is also a valve spring which urges the elastomeric disk against the valve seat in a sealing relationship. When the stem of the valve is actuated, the disk pivots on a section of the valve seat and unseals another section of the valve seat.
The invention further contemplates bi-directional ink jet color printing without hue shift through positioning the regulators and print heads with respect to the printer carriage.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.